1. Field of the Invention
The invention relates to glow plugs which have a heating rod with an inner pole with glow and optionally control element(s), the heating rod being electrically insulated relative to the glow plug body, and a process for producing these glow plugs.
2. Description of the Related Art
Making glow plugs with a metal heating rod with an inside pole and with metal glow plug bodies such that between the wall of the heating rod and the glow plug body heat-resistant electrical insulation in the form of a glass seal is inserted is known from general practice. Here the metal glow plug body and the metal heating rod with their respective terminal as well as the glass tubes located between the glow plug body and the heating rod are inserted into a graphite mold and heated in a furnace until the glass melts. The terminals are electrically insulated from one another by ceramic tubes. After cooling and solidification of the molten glass the glow plug body is fixed insulated against the heating rod and the terminals. In this complex production process treatment must take place in a furnace, by which among others also the use of heat-resistant materials is necessary. Construction precision and operating quality cannot be easily ensured in this process, since for example during the process distortion of the components is possible. The just described technical effort of producing glow plugs of this type corresponds to the economic cost.
It has likewise been proposed that at least the heating rod be produced from ceramic material; in doing so in a multistage production process for which special tools and devices are necessary the ceramic must be shaped, compounded, fired and ground into the heating rod. In addition, the ceramic heating rod must be connected to a support tube and can only be built into a finished glow plug in conventional installation.